1. Field of the Invention
This invention relates generally to heating apparatus, and particularly to a sensor for detecting the heating condition of an object to be heated.
2. Prior Art
In a heating apparatus, such as a microwave oven used for cooking, food to be cooked is apt to be overheated because of very high heating speed. Therefore, it is necessary to detect the heating condition of the heating object, such as food, so as to stop heating or reduce heating power.
One conventional example of a sensor used for detecting heating condition is shown in U.S. Pat. No. 3,839,616. However, the technique disclosed in this patent requires relatively complex structure since the variation in the electrical resistance of a sensing element is detected for the detection of heating condition. Furthermore, the sensing element has to be kept clean because it is apt to become dirty.
Another conventional example is disclosed in U.S. Pat. No. 3,467,804. However, this technique also requires complex structure since acoustic impedance is detected by way of an acoustic transmitter and an acoustic receiver, and involves additinal complexities to detect various pieces of food having different shapes and dimensions.